A large-sized crane has a long boom and the boom bends greatly. Accordingly, a large error arises when an attitude of the boom is detected by a conventional length detector and/or angle detector. Thus, calculating a boom attitude by means of a GPS is proposed (see, Patent Literature (hereinafter, abbreviated as PTL) 1, for example).
However, calculating a boom attitude by means of a GPS includes a problem that a boom attitude cannot be detected if radio waves from GPS satellites are interrupted. When a boom attitude cannot be detected, a safety apparatus such as an overload prevention device that utilizes a detected boom attitude does not function, and thus crane operation has to be stopped.